In a system or network carrying AC current, the current, as measured at any particular point in the system, may lead, or lag, the voltage measured at the same point by some phase difference. To the extent to which current and voltage are out of phase, the system may be said to be carrying reactive power at that point, in addition to Active (or true) power, P, which is the product of the voltage and in-phase component of the current carried at a specified point in the system. Reactive power is the product of the voltage and quadrature component of the current, and is commonly designated by the letter Q, as it will be herein. Reactive power delivery to the load of a system, within specified limits, is crucial to the maintenance of voltage control, owing to the fact that loads may contain significant non-ohmic components, and thus impose reactive power demands on the system. Reactive power issues are discussed at length in Principles for Efficient and Reliable Reactive Power Supply and Consumption, Federal Energy Regulatory Commission Staff Report (2005), which is incorporated herein by reference.
Definitions: A “reactive controller,” as the term is used herein, refers to a controller which controls either a switched reactor or a switched capacitor, either of which may be referred to herein as a “switched reactive element.” A “reactive controller” may, additionally, control other power network elements, but it must control a switched reactor or switched capacitor to fall within the definition used herein. Until today, the only kind of reactive controller that anyone has ever thought of is a “master reactive controller” that is the sole reactive controller for an entire power system. There has been no suggestion in the art of a reactive controller on the reactive power generator side of a series impedance between a reactive power generator and a distributed generation plant, because it has been assumed that reactive control must encompass the entire system.
The term “master reactive controller,” as used herein and in any appended claims, refers to a reactive controller that controls the reactive power at a centralized location within a power system. Such reactive controllers, in the context of power generation by wind power generators grouped into wind farms, may be referred to as “wind farm controllers,” a term used in U.S. Pat. No. 7,166,928 (“Larsen”) and in US Published Application US 2010/0025994 (“Cardinal et al.”), which are incorporated herein by reference. Larsen teaches that a relatively fast voltage regulator at an individual turbine generator is adjusted by a relatively slow wind-farm level reactive power regulator, which is an example of what is referred to herein as a “master reactive controller.” The voltage regulators of Larsen have no reactive control capability, and are, thus, not reactive controllers, as the term is used herein. Cardinal et al. teach wind farm controllers and an “intra-area master reactive controller.” The wind farm controller and the intra-area master reactive controller are all “master reactive controllers” in the sense in which the term is used herein.
Reactive controllers used today may be referred to as “standard reactive controllers.” Reactive power control is described, for example, in published US Patent Application 2003/0173938, to Trainer, incorporated herein by reference. An example of a standard reactive controller is a reactive power management system (RPMS) used in the context of a wind farm. All such standard reactive controllers are master reactive controllers. An example of a reactive controller is a Dynamic Reactive Power Controller, available commercially from Advanced Energy Conversion, LLC of Schenectady, N.Y., for use with external capacitor stages. All standard reactive controllers suffer from limitations including response time and dynamic range, for example.
The foregoing limitations are particularly acute when wind generators are connected to the power grid, since wind generation is necessarily volatile, insofar as it cannot be scheduled on demand. It quickly became apparent to the grid operators that there was an adverse effect from reactive power (measured, typically, in megavolt amps reactance, or MVAR) absorbed from the grid by early wind generators with inductive generators. Power factor correcting capacitors had to be installed at or near the point of grid interconnection (POI) of each wind generator, and, in some cases, utilities installed capacitors to make the correction.
Supplying the required amount of MVAR with capacitors from the generation side of an interconnection substation transformer, however, cannot be achieved without causing an unacceptable voltage rise to the generator side. Moreover, the control of both steady state and dynamic response of capacitive MVAR cannot be achieved satisfactorily with traditional control techniques, creating an urgent need for a remedy.
In addition, certain distributed generators in current use can vary their reactive power for independently controlling the voltage levels measured at the AC mains of each individual distributed generator. A result of the operation of prior art systems is that each generator may be called upon to reduce reactive output to lower the voltage if the voltage exceeds the upper level of the nominal operating band, even if additional reactive output is desirable. Conversely, each generator may increase reactive output to increase the voltage if the voltage falls below the lower level of the nominal operating band. As used herein and in any appended claims, the terms “AC mains,” or “local mains,” as applied to a distributed generator, refers to the AC voltage measured at the transformer of each wind turbine generator.
Standard reactive controllers, such as reactive power management systems (RPMSs), may turn power generation sources (wind turbines, solar plants, etc.) on and off or regulate sources of dynamic dynamic reactive power in coordination with other power regulators and may include power factor control. Such systems, however, operate under well-known constraints, however such constraints may remain uncharacterized and may be either steady state or dynamic and may or may not be reported per United States of America Federal Energy Regulatory Commission's rule 888 or 889 or the Security Constrained Economic Dispatch or the Interconnection Customer's Large Generator Interconnection Agreement (LGIA) in part or related to the Open Access Transmission Tariff (OATT). In particular, since the MVAR supply of the generator side of an interconnect substation transformer may cause an overvoltage condition, a remedy involving a Onload load tap changer or UnderLoad tap changer (OLTC/ULTC) on the interconnect transformer might be considered, however that solution is both costly and suffers from slow response time that can give rise to severe equipment damage. In addition, if the OLTC/ULTC is a position as to maximize the transformation ratio across the transformer, then the voltage at the point of interconnection falls, the chances of a dynamic reduction in reactive power support increases. In addition if the OLTC/ULTC Transformation ratio is at a minimum, and the voltage at the point of interconnection increases chances of a dynamic reduction in reactive power support increases.
General principles of electric power distribution may be found in Short, Electric Power Distribution Handbook, (CRC Press, 2004) and in Ackermann, Wind Power in Power Systems (Wiley, 2005), both of which books are incorporated herein by reference. The definitive treatises describing the state of the art in reactive controllers include Miller, Reactive Power Control in Electrical Systems (Wiley, 1982), and Blaabjerg et al., Power Electronics for Modern Wind Turbines, (Morgan & Claypool, 2006), both of which are incorporated herein by reference.